libfetch: Retry with proxy auth when server returns 407

PR: 220468
Submitted by: Egil Hasting <egil.hasting@higen.org> (based on)
Reviewed by: kevans, kp
Approved by: kp
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29533